Ex ce rpt from
Geologic Trips
San Francisco and the Bay Area
by Ted Konigsmark
ISBN 0-9661316-4-9
GeoP ress
All rights reserved. No part of this book may be reproduced without written
permission in writing, except for critical articles or reviews.
For othe r geologic trips see :
www.geologictrips.com
Trip 1. SAN FRANCISCO
Alcatraz Island
Fort Point
Palace of the Legion of Honor
Cliff House
Twin Peaks
Golden
Gate Bridge
B
2 Miles
Al
ca
tr a
zt
M
ar
in
Hu
nte
rs
P
He
a
dl
C
ity
Sa
te
r
Bridge
er
ra
ne
me
lan
ge
ra
ne
e
o
g
le
ol
un
Br
in
ge
a
nt
an
el
m
ou
M
te
rra
ne
M
Fo erc
rm ed
at
io
n
s
C
n
A
an
d
oin
t
Bay
Geologic Map
Twin Peaks
M
ge
an
el
m
ge
lle
n
Co
ai
ty
nt
Ci
ou
M
o e
un an
Br terr
n
Sa
A
ar
in
H
ea
dl
Hu
n
an
ds
te
rs
te
rra
ne
Al
Po
i nt
m
el a
ng
ca
tra
zt
er
ra
ne
e
Cross Section
The map and cross section show the distribution of the five major
Franciscan rock units that underlie San Francisco. Examples of each of
these rock units will be seen during the geologic trip to San Francisco.
46
B
Trip 1.
SAN FRANCISCO
The Franciscan
San Francisco rests on a foundation of Franciscan rocks. Although there
are many different types of Franciscan rocks, they share a common
parentage in that they were all brought together in the Franciscan
subduction zone during the collision between the Farallon plate and the
North American plate in Jurassic and Cretaceous time from 65 to 175
million years ago.
The Franciscan rocks in San Francisco have been divided by geologists
into five different rock units. You will see examples of each of these
rock units during this geologic trip to San Francisco. The trip can be
easily completed by car in one day. These are the places you will visit:
Alcatraz Island: Alcatraz Island is made up of thick sandstones of the
Alcatraz terrane. At Alcatraz you will learn where these sandstones
came from and why they got sentenced to Alcatraz for life.
Fort Point: The fort at Fort Point is built on a large block of serpentine
within the Hunters Point melange. This serpentine is a piece of oceanic
crust from the lower part of the Farallon plate.
Palace of the Legion of Honor: Here you will see a landslide that was
responsible for closing a half-mile section of El Camino del Mar. The
landslide was caused by the weak rocks of the City College melange.
Cliff House: Cliff House is built on thick sandstone beds of the San
Bruno Mountain terrane. These sandstones were deposited in a deep
oceanic trough along the Franciscan subduction zone during upper
Cretaceous time.
Twin Peaks: The Twin Peaks consist of pillow basalt and red chert of
the Marin Headlands terrane. These rocks once formed the upper part of
the Farallon plate when the Farallon plate was several thousand miles
west of San Francisco.
47
The Franciscan rocks in San Francisco occur in five northwest-trending
bands that cut diagonally across the city. If you were to trace these rocks
deep below the earth’s surface, you would find that the rocks form
layers that are stacked like giant pancakes on a platter tilted to the
northeast. The bands of rocks that you see on the surface are the uptilted
edges of these rock layers.
This is not an ordinary platter of pancakes. In this stack, the oldest layer,
the Alcatraz terrane, is on the top of the stack and the youngest layer,
the San Bruno Mountain terrane, is on the bottom. The younger rock
units were stuffed under the older rock units as the rock units entered the
Franciscan subduction zone. It’s as if the younger pancakes were stuffed
under the older pancakes on the platter.
After the subduction was completed, the Franciscan rocks were uplifted
and eroded. Rocks that had once been buried at depths of up to 30 miles
in the subduction zone were eventually exposed at the surface of the
ground. These rocks now make up the hills of San Francisco and many
of the islands in the bay.
FRANCISCAN ROCKS
San Francisco
Locality
Rock Unit
Age
Description
Alcatraz Island
Alcatraz
terrane
Lower
Cretaceous
Graywacke sandstone, little or
no K-feldspar
Fort Point
Hunters
Point
melange
Jurassic
Cretaceous
Melange: large blocks of
serpentine in soft clay and
serpentine matrix
Twin Peaks
Marin
Headlands
terrane
Jurassic
Cretaceous
Pillow basalt, red chert and
sandstone, typically in thin
fault slices
Palace of the
Legion of Honor
City College
melange
Jurassic
Cretaceous
Melange: blocks of basalt,
chert, and sandstone in soft
clay and serpentine matrix
Cliff House
San Bruno
Mountain
terrane
Upper
Cretaceous
Graywacke sandstone,
contains K-feldspar
48
FRANCISCAN SUBDUCTION ZONE
Melange
Zone of
underthrusting
Terrane
Depth in miles
0
10
North
American
plate
Farallon plate
1
20
30
160 Million year ago
Depth in miles
0
10
Farallon plate
4 3
2
North
American
plate
1
20
30
65 Million year ago
In this model of the Franciscan subduction zone, terrane
went into the
subduction zone first, followed by terranes 2, 3, and .. These terranes
entered the subduction zone during Jurassic and Cretaceous time. At the
end of the Cretaceous, the rocks in the subduction zone were uplifted.
Eventually, rocks that had been buried at depths of 30 miles were exposed
at the surface by erosion of the overlying rocks.
49
Alcatraz Island
Alcatraz Island is the best place in the Bay Area to see the Alcatraz
terrane of the Franciscan. The Alcatraz terrane is characterized by thickbedded sandstones. These sandstones form many of the hills that give
downtown San Francisco its unique character, including Telegraph Hill,
Russian Hill, Rincon Hill and Nob Hill. However, in San Francisco, the
Alcatraz sandstone is almost everywhere covered by buildings, streets
and cable car tracks, so that good exposures are hard to find.
All of the rocks that you see on Alcatraz Island belong to the Alcatraz
terrane. There are numerous good exposures of these rocks, but access is
limited by the Park Service and the sea gulls. The trick is to find
outcrops of the Alcatraz sandstones that are accessible. Two geologic
sites will be described on this trip, the Officers Club and the Cellblocks.
Along the path near the Officers Club you will examine the sandstone in
detail and learn where the sand grains came from. At the Cellblocks you
will see the thick and massive character of the sandstone beds that has
made them so resistant to erosion.
To get to Alcatraz Island, take a Red and White Ferry from Pier 41 at
Fisherman’s Wharf (Phone 1-800-BAY CRUISE). Departures are at 30minute intervals throughout the day and tickets go on sale at 8 AM.
During the summer you should make advance reservations.
In the late 1800’s and early 1900’s, Alcatraz Island was controlled by
the U.S. Army, which used it as a fort and later as a military prison. The
federal government took control in 1934 and used it until 1963 as a
high-security penitentiary for criminals considered too dangerous to be
held in conventional jails. In 1972 Alcatraz became part of the Golden
Gate National Recreation Area. The GGNRA now provides an excellent
self-guided tour of the penitentiary, and you should go on this tour while
you are on “The Rock.”
During the glacial stages of the Pleistocene, when sea level was
exceptionally low, there was no water in San Francisco Bay and
Alcatraz Island was connected by land to San Francisco. Alcatraz at
these times was just another hill like Telegraph Hill, Nob Hill, Russian
Hill, and Rincon Hill. During these periods of low sea level, the
Sacramento River flowed westward through Raccoon Strait between
Angel Island and the Tiburon Peninsula and then through the Golden
Gate. There is still a deep channel in the bay through Raccoon Strait that
follows this old river channel.
50
ALCATRAZ ISLAND
ub
Cl
Officers
Club
Geologic Site
100 Feet
Yard
cise
Exer
Chapel
Sally Port
D
rry
Fe
ks
loc
llb
Ce
k
oc
s
ck
rra
Ba
Path
Cellblocks
Lighthouse
Warden’s House
Alcatraz Island consists of sandstones of the Alcatraz terrane. These
rocks form many of the hills in downtown San Francisco, but the best
exposures are on Alcatraz Island.
The steep cliffs on the southwest shore of Alcatraz Island are formed from
thick beds of the Alcatraz sandstone.
51
Officers Club
Follow the path from the ferry dock past the Sally Port and Chapel to
the first switchback at the Officers Club. At this switchback you will see
outcrops of the Alcatraz sandstone in the cuts along the path. If you look
at the sandstone in detail you will see that it forms layers a foot or so
thick and that these layers dip steeply to the east. The sandstone is light
yellow-brown in color and feels like sandpaper when you rub it. If you
examine the sandstone with a magnifying glass, you will see that it is
composed of many small grains that are cemented together. It’s like
beach sand, only much harder. Most of the grains are about the size of a
pinhead and are sharp and angular. Under a microscope, you would see
that there are several different types of sand grains. Most of the grains
are milky, opaque feldspar. About a third of the grains are clear, glassy
quartz. The other grains include shale, volcanic rocks, metamorphic
rocks, and red chert. The spaces between the sand grains are filled with
clay and very small grains of other minerals and rocks.
The sand grains were derived from the many different types of rocks
that were being eroded along the margin of the North American
continent during early Cretaceous time. These rocks were broken into
small sand grains by weathering and erosion. The sand grains were then
carried by rivers to the ocean and deposited on the sea floor. The
transportation and deposition of the sand grains took place rapidly. The
sand grains would have been rounded if transportation had been for long
distances. Also, the grains composed of soft volcanic rocks and shale
would not have survived long transportation. The clay, mica, and
chlorite that make up the matrix of the sandstone are mainly weathering
products formed by chemical alteration of the less resistant rocks in the
source area. Dirty sandstones such as these are referred to by geologists
as graywackes. Most of the sandstones in the Franciscan are
graywackes.
Cellblocks
To get to this locality, follow the path to the west side of the island,
where you can see the west side of the Cellblocks. The Cellblocks are
built on very thick beds of Alcatraz sandstone. These thick sandstone
beds form the steep cliffs on this side of the island. Note that the
sandstones exposed in these cliffs form layers about 10 to 20 feet thick.
When the sandstone was deposited on the sea floor during Lower
Cretaceous time, it was deposited one layer at a time, each layer
covering the previous layer on the sea floor. These layers of sand were
horizontal when they were originally deposited. The tilting occurred
during subsequent uplift and folding of the rocks. Probably most of the
52
tilting occurred when the Alcatraz sandstone was carried into the
Franciscan subduction zone a few million years after it was deposited.
Where these layers are especially thick, they are very resistant to
weathering and erosion. These thick sandstones have given us many of
the hills in downtown San Francisco.
The wall at the right is part of the exercise yard at Alcatraz. This wall is
built on massive sandstone cliffs of the Alcatraz terrane in the center of the
photo.
Telegraph Hill: If it is not convenient to get to Alcatraz Island, you can
also see some exposures of the Alcatraz sandstone at Telegraph Hill. To
do this, go to Sansome and Union Street on the east side of Telegraph
Hill. On the west side of this intersection you will see a large cliff of
Alcatraz sandstone. Years back, this cliff was the face of a rock quarry.
The sandstone in the cliff is very massive; that is, it is not interbedded
with soft shale beds. It is because this sandstone is so massive that
Telegraph Hill has resisted weathering and erosion.
You can see other exposures of the sandstone along the pathway to Coit
Tower that leaves from the intersection of Lombard Street and
Telegraph Hill Blvd. Do not be fooled by the large red boulders along
the road at the top of Coit Tower. These rocks are jasper, and were
carried here by truck from somewhere else.
53
Fort Point
The point of land that guards the south side of the Golden Gate is a large
block of serpentine that lies within the Hunters Point melange of the
Franciscan. The fort at Fort Point was built on this block of serpentine.
Another large block of serpentine supports the south tower of the Golden
Gate Bridge. The Hunters Point melange contains a number of these
large blocks of serpentine. During the trip to Fort Point you will visit the
parking lot at Fort Point and the bluff at Baker Beach. At the parking lot
you will see part of the serpentine block that underlies the fort and at the
Baker Beach bluff you will see the Hunters Point melange where it
contains smaller blocks of serpentine and many other rocks.
To get to Fort Point from Fisherman’s Wharf, go west on Bay Street to
Van Ness Ave., turn left and go three blocks to Lombard Street, turn
right and go 1.2 miles to Lincoln Blvd. Turn right on Lincoln Blvd. then
right on Long Ave. Follow Long Ave. and Marine Drive to the parking
area at Fort Point. Fort Point National Historic Site is within the Golden
Gate National Recreation Area. The fort is open daily 10 AM to 5 PM.
For information, phone 415-556-1693.
Golden
Gate Bridge
FORT POINT
Geologic Site
Fort Point
1/4 Mile
Parking Lot
sy
e
Av
eld
Fi Lincoln
Lin
co
Blvd
ln
Blv
d
Path
is
Cr
Hunters
Point m
elange
Rd
1
10
Baker B
each
nt
ha
e
Av
ng
c
er
M
ay
hw
ig
H
Baker
Beach
Bluff
Dr
Toll
plaza
Lo
Bluff edge
Marin
e
Fort Point was built on a large block of serpentine that lies within the
Hunters Point melange of the Franciscan.
54
M as
on S
t
Parking Lot
The large serpentine block that forms the foundation for the fort is
exposed in the small hill at the parking lot next to the fort. The
serpentine in this exposure is light green, flaky, and intensely foliated
with many curved surfaces along which the serpentine tends to break.
The foliations wrap around smaller blocks of hard serpentine that are
darker green. The serpentine feels very slippery and has a waxy luster.
Some of the dark green blocks appear as if they have been polished.
There are other good exposures of this serpentine in the road cuts all
along Marine Drive.
Serpentine is formed at spreading centers. It consists of ultramafic rocks
from the earth’s mantle that were altered by hot seawater. The hot
seawater reached the ultramafic rocks through cracks formed in the
earth’s crust during the spreading process. The serpentine was carried
away from the spreading center as the oceanic plate moved away from
the spreading center. Serpentine forms the lower part of the earth’s
oceanic crust worldwide.
Fort Point, which lies under the south approach to the Golden Gate Bridge,
is framed by a special arch on the bridge that was built to preserve the fort.
The serpentine of the Hunters Point melange is exposed in the small hill at
the parking lot to the left of the fort.
55
Serpentine is very common in the Franciscan throughout the Coast
Ranges. In some places the serpentine occurs as small, isolated blocks
and in other places as mountain-sized slabs. The most extensive
exposures of serpentine are along the eastern edge of the northern Coast
Ranges where a large slab from one- to five-miles wide and 70 miles
long separates the Franciscan from the sedimentary rocks that were
deposited in the Great Valley. The sedimentary rocks in the Great
Valley were apparently deposited on this oceanic crust, then the
Franciscan was thrust under this oceanic crust during subduction.
Many of the smaller isolated blocks of serpentine in the Franciscan
represent pieces of the oceanic crust that were broken up and then
squeezed upward through the overlying host rock like watermelon
seeds. The serpentine in these blocks tends to break into small, dark
green fragments with curved polished surfaces. The polished surfaces
have tiny grooves that formed when the fragments rubbed against each
other as the serpentine was squeezed through the rocks. Wherever you
see serpentine on land, you are looking at a piece of oceanic crust that is
out of place and was somehow injected into or squeezed onto the land.
The large blocks of serpentine in the foreground lie within the Hunters
Point melange. The rocks in the surf zone have been excavated from the
Hunters Point melange during erosion of the Baker Beach bluff.
56
Baker Beach Bluff
To get to the bluff at Baker Beach from Fort Point, return on Long Ave.
to Lincoln Blvd., turn right on Lincoln Blvd., and then take the road that
goes to the Toll Plaza. Follow this road under the Toll Plaza to
Merchant Road and park in the large parking area on Merchant Road
Take the path to the Coastal Trail, then follow the trail north until you
go down a short wooden staircase. At the base of the stairs leave the
Coastal Trail and follow the path to the edge of the bluff.
The bluff here is made up of the Hunters Point melange. The melange
consists of finely crushed serpentine, shale and sandstone with large
random blocks of serpentine and many other rocks. There are many
landslide deposits along the bluff and it is difficult in some places to
distinguish between the landslides and the melange. Along the shoreline
are numerous large boulders that have accumulated after being eroded
from the melange. These boulders include sandstone and shale, basalt,
chert, greenstone, serpentine polished by wave action, serpentine with
small veins of asbestos, red jasper, yellow jasper, intensely deformed
sandstone cemented by quartz and calcite veins, and metamorphosed
gabbro. Since this beach is somewhat secluded, on a sunny day you may
see much more exposed than just these rocks.
The Hunters Point melange occurs in a wide belt that trends southeast
through San Francisco from Fort Point to Hunters Point. Because the
clay in the melange is soft, there are few exposures of the clay along the
outcrop belt. However, the large serpentine blocks within the melange
are hard and therefore typically form small hills along the outcrop belt.
During construction of the Golden Gate Bridge there was some
controversy concerning whether the serpentine block under the south
tower would provide adequate support for the bridge. To settle this
dispute, Andy Lawson, who was then a Professor of Geology at U.C.
Berkeley, was lowered into the deep foundation hole that had been dug
into the serpentine. Professor Lawson proclaimed that the rock was
satisfactory and construction of the bridge continued. Another large
block of serpentine forms the foundation of the New Mint building at
Dubose and Church Streets. The mint building is built on a small hill,
and the serpentine can be seen directly under the mint building. This is
probably the best exposure of serpentine in San Francisco. The
serpentine under the mint consists of blocks of serpentine a couple of
feet in diameter that are surrounded by highly foliated serpentine. Other
large blocks of serpentine form Portrero Hill and Hunters Point.
57
Palace of the Legion of Honor
Some years back you could drive west along El Camino del Mar from
the Palace of the Legion of Honor to Ocean Lookout Park. This section
of El Camino del Mar was built on the City College melange of the
Franciscan and is now abandoned because of landslides caused by the
weak and slippery clay in the melange. During the geologic trip to the
Palace of the Legion of Honor you will visit one of these landslides.
To get to the Palace of the Legion of Honor from Fort Point, go west on
Lincoln Blvd. to El Camino del Mar, and follow El Camino del Mar to
Legion of Honor Drive. Do not turn left on Legion of Honor Drive, but
continue west on El Camino del Mar about 200 yards until you reach the
barrier across El Camino del Mar. This section of the road is now used
as a parking area. Park here and go the road barrier.
PALACE OF THE
LEGION OF
HONOR
Bluff
edge
Geologic Site
Palace
of the
Legion
of
Honor
Landslide
Lands
end
1/4 Mile
Road
barrier
ge
lle
Co
o del
Le
gio
no
fH
on
or
Dr
Clement St
me
r
Ma
Geary Blvd
ge
The City College melange consists of soft clay that contains hard blocks of
basalt, serpentine, sandstone and other rocks. The soft clay in the
melange has resulted in landslides that have led to abandonment of a halfmile section of El Camino del Mar.
58
34th St
lan
Cliff
House
de
sli
nd
y
Cit
in
El Cam
Pacific
Ocean
road
Abandoned
del
Ma
r
no
mi
Ca
El
La
Ocean
Lookout
Park
Landslide
To get to the landslide, take the path going west from the road barrier at
the end of El Camino del Mar. Immediately past the barrier, the path
goes down a 100 or so steps and then crosses a small bridge. The steps
are on the landslide and the bridge is in a gully that was formed by the
landslide. Near the bridge there are large chunks of asphalt from the
abandoned road, and just beyond the bridge there is a large section of
asphalt with the white mid-line stripe still on the asphalt. These pieces
of the roadway have slid downslope about 50 feet courtesy of the
landslide.
If you have the time, continue west on the abandoned section of the
road. By keeping track of the pieces of asphalt you can see where
several sections of the road are missing. In less than half a mile you will
reach the road barrier at Ocean Lookout Park, where El Camino del Mar
continues to the west. While at Ocean Lookout Park you can see the
memorial to the U.S.S. San Francisco, which was lost during the battle
of Guadacanal. Just north of the memorial you will find some wooden
steps that go down to the Coastal Trail. Go down these steps, then along
If you travel west on El Camino del Mar, the road ends abruptly just beyond
the Palace of the Legion of Honor. The section of the road beyond the
barrier in the photo was removed by a landslide in the soft rocks of the City
College melange. The pathway to the right of the barrier goes down the
landslide and then follows the abandoned road to Ocean Lookout Park,
where you will find the continuation of El Camino del Mar.
59
the Coastal Trail a few feet to the east to where you can get a view
along the bluff to the east toward Lands End. Be careful and don’t go
on the bluff where it is unsafe.
Most of the rocks exposed along the bluff between Ocean Lookout
Park and Lands End are the City College melange. The melange
consists of hard blocks of serpentine, schist, gabbro, basalt, chert, and
sandstone in a soft matrix of fine clay and serpentine. As this section of
the bluff has been eroded by the ocean waves, the hard boulders in the
melange have remained behind and accumulated at the base of the
bluff. These boulders now protect the bluff from rapid erosion.
In the early 1880’s, a rail line had followed this same route and took
people from downtown San Francisco to the Cliff House. The rail line
was abandoned at the turn of the century because of maintenance
problems related to the unstable roadbed. The fine clay matrix of the
melange tends to hold the water near the surface and therefore becomes
very slippery when wet. The clay then flows downhill, taking with it
the roads, buildings, and whatever else has been built on the melange.
The clay matrix in the melange provides weak and slippery rocks
wherever exposed in San Francisco. The rocks along the bluff are
especially susceptible to landslides because the rocks are on a very
steep slope. This combination of weak rocks, steep slope and heavy
rainfall conspired to do away with this section of El Camino del Mar.
The City College melange cuts across San Francisco from Lands End
through Golden Gate Park and McLaren Park to San Francisco Bay just
south of Candlestick Park. The melange was formed along the zone of
thrusting between the rocks of the San Bruno Mountain terrane and the
Marin Headlands terrane while the rocks were in the Franciscan
subduction zone. The fine clay matrix of the melange was formed from
rocks that were thoroughly ground up by the thrusting. The blocks in
the melange are pieces of hard rock that survived the grinding.
Melanges are very common in the Franciscan throughout the Coast
Ranges of Northern California. Large random boulders on rolling
hillsides are a tip-off that the rocks may be a melange. The boulders are
very hard and stand out from the soft greenish-gray or bluish-gray clay
matrix, which is seldom seen. The clay matrix does not have the thin
layering of typical Franciscan shale. Landslides are especially common
where the melange occurs on a steep slope.
60
These steps go down the landslide at the end of El Camino del Mar. The
landslide removed a several-hundred-foot section of El Camino del Mar.
Pieces of the old asphalt road surface can be seen in the landslide debris
near the base of the steps.
The small islands and rocks that can be seen along the shoreline east of
Ocean Lookout Park represent hard blocks of serpentine, basalt, chert and
sandstone that have been eroded from the soft clay matrix of the City
College melange.
61
Cliff House
The sandstones of the San Bruno Mountain terrane were deposited as a
series of submarine fans that filled a deepwater trough that had formed
along the Franciscan subduction zone during late Cretaceous time. At
the Cliff House and nearby Sutro Baths you will see some of these
deepwater sandstones. The Cliff House was built on thick sandstone
beds that had been deposited in the central parts of one of the submarine
fans. The Sutro Baths are located in thinly bedded sandstone and shale
that had been deposited along the edges of the fan.
To get to the Cliff House from the Palace of the Legion of Honor,
follow Legion of Honor Drive to Geary Blvd. and turn right. Follow
Geary Blvd. to Fortieth Avenue and then continue on Point Lobos Ave.
to the Cliff House. Since the 1850’s the Cliff House and its
predecessors, including the Ocean House and Seal Rock House saloons,
have been prime tourist destinations. The Cliff House is in the Golden
Gate National Recreation Area. For information from the Park Service,
phone 415-556-8642.
El
Bluff
edge
Parking
Merrie Way
Sutro Baths
Sutro
Baths
Ruin
nt
Poi
Cliff House
Seal
Rocks
ar
el M
od
min
Ca
Path
Ave.
Lobos
Geary Blvd.
Sutro
Heights
Park
G
at
re
H
h
ig
Pacific
Ocean
1/4 Mile
ay
w
Geologic Site
each
Ocean B
CLIFF HOUSE
San Bruno
Mountain
terrane
The Cliff House and the Sutro Baths are built on sandstones of the San
Bruno Mountain terrane of the Franciscan.
62
Cliff House
To see the thick sandstones of the San Bruno Mountain terrane, go
down the steps leading to the visitor center on the observation deck at
the Cliff House. The sandstone beds are exposed in the cliff below the
observation deck. Continue south along the deck to where some of the
sandstone is exposed along the path. The sandstone is light yellow
brown, and the beds are very thick. There are almost no layers of black
shale between the sandstone beds. The sandstone appears similar to the
massive sandstone below the cellblocks at Alcatraz. However, there are
a couple of differences. The sandstones of the San Bruno Mountain
terrane are younger than the sandstones of the Alcatraz terrane. They
were formed in late Cretaceous time during the later stages of
subduction, whereas the rocks in the Alcatraz terrane were formed in
early Cretaceous time during the earlier stages of subduction. Also, the
sandstone in the San Bruno Mountain terrane contains K-feldspar, a
variety of feldspar that is rich in potassium. The K-feldspar was derived
from weathering of granitic rocks in the ancestral Sierras. These
granitic rocks had been formed in the Franciscan subduction zone.
There is almost no K-feldspar in the sandstones of the Alcatraz terrane
since the Alcatraz sandstones were formed in the early stages of
subduction before the Sierra granite had been subjected to erosion.
The massive sandstones of the San Bruno Mountain terrane are highly
resistant to erosion and provide a firm foundation for the Cliff House.
63
Sutro Baths
The Sutro Baths were built by Adolph Sutro, a popular San Francisco
philanthropist. The indoor bathhouse was opened in 1896 and had
seven separate pools spread across a three-acre site. The baths were
enormously popular into the 1920’s, but declined thereafter, and were
destroyed in a fire in 1966.
The ruins of the Sutro Baths lie immediately north of the Cliff House.
To get to the ruins from the Cliff House, go east on Point Lobos Ave.
one block to Merrie Way, turn left, and park in the parking area. Take
the path that leads to the tunnel immediately north of the ruins. At the
entrance to this tunnel there are good exposures of interbedded
sandstone and shale of the San Bruno Mountain terrane. The sandstone
beds are similar to the sandstone at the Cliff House except that the beds
are thinner, from one- to ten-feet thick. The hard sandstones form
ridges that stick out from the soft black shale that occurs between the
sandstone beds. Because the sandstone beds here are thin and
interbedded with the soft shale, they are eroded more easily than the
massive sandstone under the Cliff House. The cliff thus makes a
reentrant in these softer rocks.
The sandstone that makes up the San Bruno Mountain terrane extends
southeast from Cliff House through San Bruno Mountain to Oyster
Cove on San Francisco Bay. There are extensive exposures of the
sandstone in San Bruno Mountain State Park and San Bruno Mountain
County Park.
From studies of sandstones in many other parts of the world, we know
that the sandstones in the San Bruno Mountain terrane were deposited
in deep ocean water by turbidity currents, dense mixtures of water and
sediments that flow down the slope of the sea floor like underwater
avalanches. A huge amount of sand and other sediments was required
to form the widespread sandstones of the San Bruno Mountain terrane.
Most of the sediments that make up the San Bruno Mountain terrane
were derived from the weathering and erosion of granitic rocks of the
ancestral Sierras. These sediments were carried westward from the
Sierras by ancient rivers to the ocean, and then carried along the coast
until they reached a submarine canyon. The sediments accumulated on
the sea floor near the head of the submarine canyon until they became
unstable, perhaps because of a violent storm or earthquake. When the
sediments became unstable, they slumped, mixed with ocean water,
and formed a turbidity current. Since the turbidity current was heavier
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than the ocean water, it flowed down the submarine canyon until it
reached the base of the canyon. At the base of the canyon the turbidity
current would spread out onto a large fan in deep water and the sand and
mud were deposited on the surface of the fan. The sand settled out of the
turbidity current rapidly, and the fine mud settled out slowly at the end of
the flow.
Each turbidity current flow typically resulted in deposition of a flat layer,
or “bed” of sediments, with sand at the base and mud at the top. Over
time, many beds of sand and mud accumulated on the fan in this manner,
one on top of the other, eventually forming a unit of sandstone and mud
several thousand feet thick and covering several hundred square miles.
The mud is now represented by the layers of shale that occur between the
sandstones.
The Sutro Baths lie in a reentrant along the bluff just north of the Cliff
House. Here, the sandstones in the San Bruno Mountain terrane are
interbedded with many layers of soft black shale. The interbedded rocks in
the reentrant were more easily eroded than the thick massive sandstones
under the Cliff House.
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Twin Peaks
San Francisco’s famous Twin Peaks are formed from red chert and
pillow basalt of the Marin Headlands terrane of the Franciscan. On this
trip you will park at the viewpoint at the top of Twin Peaks and then
take a short walk along the loop road to see these rocks.
To reach the Twin Peaks viewpoint from the Cliff House, go south on
the Great Highway to Sloat Blvd., turn left and follow Sloat Blvd. to
Portola Drive. Follow Portola Drive to Twin Peaks Blvd., and then
continue on Twin Peaks Blvd. to the parking area for the viewpoint.
The viewpoint is in Twin Peaks Park, and provides an excellent view of
San Francisco, looking directly down Market Street to San Francisco
Bay.
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The red chert and pillow basalt that make up much of the Marin
Headlands terrane are well exposed along the loop road at the
top of Twin Peaks. The northern peak consists of red chert and
the southern peak is pillow basalt.
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Viewpoint
Upon arriving at the viewpoint, be different from everyone else. Instead
of going to the observation area to look at the city, take a walk on the
one-way road that circles the two peaks. On this walk you will see that
the Twin Peaks are not identical twins, but fraternal twins. The southern
peak consists of pillow basalt and the northern peak consists of red
chert.
If you start from the parking area for the viewpoint on the northern peak
and follow the loop counterclockwise, you will first see the red chert in
the road cuts. The red chert appears as bands about one- to two-inches
thick. The chert is very hard and will scratch a knife blade or glass. The
hard, red chert resists weathering and therefore tends to form ridges and
hill tops. As you continue on the loop you will see that the road cuts in
the southern peak are quite different. They consist of pillow basalt,
which on these weathered exposures looks chunky and yellow-brown.
The red chert and the pillow basalt were formed along the East Pacific
Rise spreading center during Jurassic time when the spreading center
was far west of the North American continent. As the rocks were
formed, they became the upper part of the Farallon plate and were
carried hundreds of miles eastward into the Franciscan subduction zone.
You will see these same rocks on the trip to the Marin Headlands and
learn in more detail how the red chert and pillow basalt were formed.
As you leave the Twin Peaks, return to San Francisco by taking Twin
Peaks Blvd. to the north. Along this road you will see many more
exposures of the red chert that forms the northern peak.
The rocks of the Marin Headlands terrane extend as a broad band from
Baker Beach through the eastern part of Golden Gate Park to
Candlestick Hill. This band of rocks includes several hills that are
formed primarily from the red chert, including Mt. Sutro, Diamond
Heights, Bernal Heights, Mt. Davidson, and Candlestick Hill. These
hills formed where the chert was especially thick. In other areas along
the outcrop belt, where the red chert in more poorly developed, the
Marin Headlands terrane occurs as small random exposures of pillow
basalt, red chert and sandstone. These rocks can be seen at Billy Goat
Hill near Diamond Heights, the Corona Heights Playground, and Glen
Canyon Park. At Billy Goat Hill, the pillows in the pillow basalt form a
short staircase up the side of the hill.
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